Comment out all the other irrelevant code in the loop and other irrelevent code after the loop so that only this remains.
for(i=0;i<=a.Height;i++)
{<blockquote>for
(j=
0;j<=a.
Width;j++
) {<blockquote>Color c=a.
GetPixel(i,j
);</blockquote>
}</blockquote>
}
See if the error comes up again. If not, keep on adding lines until you get the error, and try to see what are the values of i and j this happens.